Differential units are commonly employed in road vehicles to divide the drive from the engine between road wheels on opposed sides of the vehicle. In four-wheel drive vehicles, differential units are employed to divide the engine drive between the front and the rear wheels and the differential unit may be arranged to divide the incoming torque unequally. The unit may be further arranged to impose a degree of slip limitation.
U.S. Pat. No. 4,821,603 in the name Russell discloses a differential unit in which an input torque is divided unequally between two output shafts by the use of a plurality of helical gears and a worm gear. The output shafts are each connected to one of the two helical output gears of different diameters. The output gears are driven to rotate by helical intermediate gears which engage with them. The helical intermediate gears are mounted on a gear carrier and rotate freely with respect to the gear carrier. The gear carrier is driven to rotate about an axis common to both output gears. The intermediate gears each engage with a worm gear. The arrangement enables some variation in the rotation of one output shaft with respect to the other whilst slip is limited since the worm wheel to helical gear mesh involves substantial amounts of sliding and the transmission of torque is comparatively inefficient.
The differential of Russell has two disadvantages. Firstly the cross-axial gearing is known to be limited in torque capacity. Secondly the differential unit lacks compactness since the differential unit in the axial direction must accommodate the diameter of the wormwheel.
EP 0130806 in the name of Quaife discloses a differential unit which comprises two sets of helical intermediate gears in direct engagement with one another, one set engaging with one output gear and the other set engaging with the other output gear. However, the disclosure teaches only a differential unit having two output gears of the same diameter as is used for splitting torque between half shafts on the same axle. The disclosure does not provide a differential capable of dividing incoming torque unequally nor does the disclosure consider such a possibility.
It is thus an object of the present invention to provide a differential unit in which an input torque is divided unequally between two output shafts by means of a gear mechanism of simple form.
It is a further object of the invention to provide a differential unit incorporating simple means for limiting slip between the output members, which means is preferably selectively adjustable.